There exist many systems for smoke and/or aerosol detection in closed (or open) spaces. Some such systems employ light sources with photodetectors. Most of these systems are not, however, concerned with average smoke/aerosol density detection, and have a single path for a light beam between its source and a detector. Other systems use reflectors to extend the path length through the space. While these last-mentioned systems do provide some indication of average density, they do it at the expense of complex optical systems, i.e., the various reflectors must be accurately aligned. The instant invention is able to determine smoke/aerosol average density with a simple and straightforward system.